Mass spectrometers capable of multiple simultaneous detection are disclosed in U.S. Pat. Nos. 4,435,642; 4,472,631; 4,638,160; and 4,924,090. Such spectrometers are also disclosed in U.S. Pat. No. 4,998,015 and in U.S. patent application Ser. No. 07/523,588 assigned to the assignee of this application.
FIG. 1 shows one example of such a mass spectrometer capable of multiple simultaneous detection. This instrument includes an ion source 1 producing ions which are separated and focused along a focal plane l according to their mass-to-charge ratios by a mass analyzer 4 consisting of an electric field 2 and a uniform sector magnetic field 3. In order to detect the separated ions simultaneously, a simultaneous detection-type ion detector 5 having spatial resolution along the focal plane l is disposed
Usually, this simultaneous detection-type ion detector 5 consists of a microchannel plate, a phosphor, and a photodiode array. The effective active surface of the greatest photodiode array presently available has a width (the dimension taken in the direction in which the ions are dispersed according to their mass-to-charge ratios) of about 50 mm, a height (the dimension taken in the direction perpendicular to the direction in which the ions are dispersed) of about 2.5 mm. However, the vertical spread of the ion beams dispersed and focused by the mass analyzer 4 is normally on the order of 10 mm at the position of the detector Therefore, the simultaneous detection-type ion detector detects only one fourth of the ions passed through the mass analyzer 4. Consequently, the sensitivity of the mass spectrometer equipped with the simultaneous detection-type ion detector is low.